1. Field of the Invention
The present invention relates to an amplification optical fiber for amplifying signals under supply of pumping light and a fiber optic amplifier including the same.
2. Related Background Art
The fiber optic amplifiers are optical devices to which a silica-based amplification optical fiber with a core region, an equivalent of an optical waveguide region, doped with a rare earth element is applied as an optical amplifying medium and in which the amplification optical fiber amplifies signals under supply of pumping light. For example, in the case of the fiber optic amplifier (EDFA: Erbium-Doped Fiber Amplifier) making use of the amplification optical fiber (EDF: Erbium-Doped Fiber) with the core region doped with the element Er as a rare earth element, when the pumping light having the wavelength of 1.48 μm is supplied into the EDF, signals in the C-band (wavelengths of 1.53 μm to 1.56 μm) or in the L-band (wavelengths of 1.57 μm to 1.67 μm) are amplified in the EDF.
For increasing transmission capacity of information, transmission is also pursued through wavelength division multiplexing (WDM) to transmit signals of multiple channels (signals of mutually different wavelengths) in a multiplex form. The multiplexed signals (WDM signals) of multiple channels used in the WDM transmission heretofore have been signals belonging to the C-band, but research has also been conducted on use of multiplexed signals belonging to the L-band in order to meet demands for further increase of capacity. Narrowing the wavelength spacing (signal channel spacing) between multiplexed signals is also under research as one of means for further increase of transmission capacity.
Since the EDFA can amplify the multiplexed signals (WDM signals) belonging to the C-band or the L-band en bloc, it is also suitable for the WDM transmission systems in the C-band or in the L-band. In this EDFA, however, there arises a problem of occurrence of four-wave mixing (hereinafter referred to as FWM) being a kind of nonlinear optical phenomena. Namely, the multiplexed signals of multiple channels amplified in the EDF have high power and narrow wavelength spacing. In addition, the mode field diameter of EDF is smaller than those of the normal transmission optical fibers. For these reasons, FWM is easy to occur in the EDF. With occurrence of FWM, there will occur waveform degradation of each signal, which will result in degradation of transmission quality.
Japanese Patent Application Laid-Open No. H10-242556 discloses the EDF for suppressing the occurrence of FWM. The EDF disclosed in this Application has the effective core diameter of 9 μm or more at the wavelength of 1.55 μm and the zero-dispersion wavelength outside the range of 1.53 μm to 1.56 μm. The EDFA incorporating this EDF can amplify the multiplexed signals of multiple channels in the C-band en bloc and suppress occurrence of FWM on that occasion.